This invention relates to optical sensors, and in particular to moire fringe displacement sensors.
In moire fringe displacement sensors, first and second gratings are disposed adjacent one to another, such that moire fringes are produced. Relative movement of the gratings causes a corresponding movement of the moire fringes, which movement is detected by monitoring the amplitude of light reaching a detector. One problem encountered with such sensors is that a constant source is required as any modulation in the strength of the source is indistinguishable by the detector from modulation caused by the movement of the moire fringes.